The present invention generally relates to electronic sensors and more particularly relates to an active sensor assembly for detecting movements.
Sensor assemblies of this general type are disclosed, for example, in WO 98/09173 and are used especially for detecting the rotational behavior of a vehicle wheel (wheel speed sensors) in slip-controlled brakes or anti-lock systems. The rotational movement is executed by a pulse generator provided in the encoder (for example, a magnet pole wheel or a steel pinion gear) and detected by an active sensor, the pick-up for measuring data of which is for example a Hall element or a magnetoresistive bridge (AMR bridge) which responds synchronously in an effect-related fashion to modulations of the flux density or the field strength caused by the pulse generator. In the active sensor, the signal voltage produced thereby is converted by means of an amplifier/trigger circuit into a binary sensor signal with two constant amplitude values, the edge changes of which are evaluated to determine the rate of motion. Because the signal voltage depends on the size of the air slot between the sensor and the encoder, it must be ensured that the air slot will not exceed a defined air slot limit value.
In an active sensor, the sensor signal prevailing at the sensor output does not indicate the actual size of the air slot due to the internal amplifier/trigger circuit. Hence, the occurrence of errors or failures cannot be excluded if the air slot, at least temporarily, exceeds the air slot limit value due to unfavorable conditions of the sensor assembly (excessive installation air slot, major variations in temperature, vibrations) and the signal voltage drops below the internal trigger threshold.
An object of the present invention is to provide a sensor assembly of the type initially referred to wherein the size of the air slot between the active sensor and an encoder and especially unallowable variations of the air slot can be detected in order to be able to generate a corresponding status signal or take other precautionary measures in good time before a possible failure of the sensor signal.
This object is achieved according to the present invention in that there is provision of a second device by which a signal voltage that depends on an air slot between the active sensor and the encoder is detected and sent to the first device for transmission as additional information.
Especially in connection with the conversion of the binary sensor signal by means of the first device into a pulse signal, this solution permits relatively simply evaluating and determining or checking the size of the air slot. This may be carried out after the installation of the sensor assembly along with regular maintenance operations.
Accordingly, the first device preferably generates a pulse signal wherein the sensor signal is coded with first current pulses and the additional information is coded with second current pulses, wherein a first current level is provided for the first current pulses and a second current level for the second current pulses. In relation to a joint reference level, the first current level is in about twice as high as the second current level.
Further, the first device preferably comprises a signal processing device which permits transmitting further additional information in the form of status signals or numerical values which e.g. contain data about the direction of rotation, temperatures, etc.